Add the ability to change search provider in the optional search bar.

I really like the option to bring back the separate search bar, but it would be much better if it was possible to pick the search provider as well (like in the address bar). Currently it's locked to the default search engine.

And yes I'm aware than I can type the associated letter before the search, such as W test to search for test in Wikipedia, but a UI for search provider picker would be nice too.